1. Field of the Invention
The present invention relates to image forming apparatuses that can print characters and figures on various kinds of paper such as a printer and a copying machine.
2. Description of the Related Art
In general, in this type of image forming apparatuses, printing on various kinds of paper having different thicknesses and the like is possible. However, a difference in a paper type affects transferability, fixability, and the like in printing. Therefore, for example, in an image forming apparatus of an electrophotographic system, improvement in an image quality is realized by, for example, changing a quantity of toner for transferring an image to paper, transfer conditions, and fixing conditions according to paper thickness.
Conventionally, an operator visually checks and manually designates paper type information such as thickness. However, for example, because this work is extremely complicated, it is proposed to automatically detect a paper type using a transmission optical sensor or the like.
For example, in JP-A-2004-277057, paper feeding mechanism that feeds paper piece by piece is provided for each paper feeding cassette that stacks and holds paper and, on an outgoing side of the paper feeding mechanism, a paper type sensor that detects type (thickness, etc.) of paper fed by this paper feeding mechanism is provided. In preparation for printing to be performed later, a leading end of one piece of paper at the top is fed from the paper feeding cassette by a predetermined quantity by the paper feeding mechanism at timing prior to an original paper feeding command and a type of the paper is detected by the paper type sensor.
In this way, it is possible to acquire paper type information at the timing prior to the original paper feeding command. This makes it possible to perform, for an image forming process of an image forming unit, proper setting using the detected paper type information before the original paper feeding command for printing is received.
In this type of image forming apparatuses, in general, plural paper feeding cassettes as paper supplying sources are provided. Pieces of paper of types different from one another are often provided in the plural paper feeding cassettes, respectively, to make it possible to select a desired paper type from these paper feeding cassettes and perform printing. In this case, pieces of paper fed from the respective paper feeding cassettes by paper feeding mechanisms are separately sent to a common paper conveying path leading to the image forming unit, and sent to the image forming unit and printed, respectively.
A case in which the conventional technique described above is applied to such image forming apparatuses will be examined. For example, it is assumed that paper is stored in a paper feeding cassette and the paper feeding cassette is set in an image forming apparatus main body. In this case, to detect a paper type in advance in preparation for printing after this, as described above, a leading end of one piece of paper is fed from the paper feeding cassette by a predetermined quantity by the paper feeding mechanism and a type of the paper is detected by the paper type sensor. Thereafter, if an original paper feeding command for the paper, the type of which is detected, is continuously issued, no problem occurs because it is possible to send the paper fed halfway to the image forming unit as it is. However, if an original paper feeding command for paper stored in another paper feeding cassette is issued, it is likely that the paper, the type of which is detected, hinders feeding and conveyance of the paper according to the original paper feeding command.
In short, in the image forming apparatus having the plural paper feeding cassettes, it is not definitely decided to which one of the paper feeding cassettes (paper types) an original paper feeding command is given. The paper feeding cassette is changed according to a print request. Therefore, as described above, it often occurs that, after a paper type of a paper feeding cassette set anew is detected, an original paper feeding command is given to another paper feeding cassette.
In this case, since the paper, the type of which is detected, stays in a state in which the leading end thereof is fed by the predetermined quantity, it is likely that the paper hinders movement of paper fed from another paper feeding cassette to a common conveying path. To prevent the paper from hindering the movement of another piece of paper, it is necessary to provide the paper type sensor in an extremely narrow portion near the paper feeding mechanism and arrange and set the paper type sensor such that the leading end of the paper in the type detected state does not project over the conveying path of another piece of paper. Therefore, the paper type sensor is subjected to restriction in terms of arrangement. Even if the paper type sensor is arranged in this way, when a quantity of paper feeding by the paper feeding mechanism is unstable, the leading end may project to the common conveying path and interfere with the movement of another piece of paper. When double feeding occurs, remaining paper may project to the common conveying path.
To surely prevent such interference, paper once fed to the sensor portion only has to be returned into the paper feeding cassette after the detection by the sensor.
However, to return the paper partially fed from the paper feeding cassette into the paper feeding cassette after the paper type detection, a complicated mechanism is required, which causes a failure. It is not preferable to return the paper once fed because a burden on the paper is heavy and a jam is caused.
Providing the paper type sensor near the paper feeding mechanism leads to misdetection of a paper type as well. The paper feeding mechanism takes out uppermost paper stacked and held in the paper feeding cassette with a pickup roller and separates and feeds the paper piece by piece with a paper feeding roller arranged on a front surface side of the paper and a separation roller arranged on a rear surface side of the paper. However, when a separating action is not sufficient, two pieces of paper may be delivered together right behind the paper feeding roller and the separation roller. Even if the two pieces of paper are delivered together, in most cases, only one piece of paper is delivered to the conveying path by an action of the separation roller. However, when the paper type sensor is provided near the separation and paper feeding mechanisms, the paper type sensor detects these two pieces of paper. Thus, for example, in the case of a sensor that detects paper thickness, misdetection is caused.